Short-Term Colony-Stimulating Factor 1 Receptor Inhibition–Induced Repopulation After Stroke Assessed by Longitudinal <sup>18</sup>F-DPA-714 PET Imaging
Cristina Barca, Amanda J. Kiliaan, Lydia Wachsmuth, Claudia Foray, Sven Hermann, Cornelius Faber, Michael Schäfers, Maximilian Wiesmann, Bastian Zinnhardt, Andreas H. Jacobs
Abstract
Studies on colony stimulating factor-1 receptor (CSF-1R) inhibition-induced microglia depletion indicated that inhibitor withdrawal allowed the renewal of the microglial compartment via repopulation and resolved the inflammatory imbalance. Therefore, we investigated for the first time the effects of microglia repopulation on inflammation and functional outcomes in an ischemic mouse model using translocator protein (TSPO) positron emission tomography (PET)- computed tomography (CT)/ magnetic resonance (MR) imaging, ex vivo characterization and behavioral tests. <b>Methods:</b><i>N</i> = 8/group C57BL/6 mice underwent a 30 minutes transient middle cerebral artery occlusion. The treatment group received CSF-1R inhibitor 1200 ppm PLX5622 chow (Plexxikon Inc.) from days 3 to 7 to induce microglia/macrophages depletion, and went back to control diet to allow microglia repopulation. Mice underwent T2w-MR imaging on day 1 and <sup>18</sup>F-DPA-714 (TSPO) PET-CT on days 7, 14, 21 and 30 post ischemia. Percentage of injected tracer dose (%ID/mL) within the infarct, contralateral striatum and spleen were assessed. Behavioral tests were performed to assess motor function recovery. Brains were harvested at days 14 and 35 post ischemia for ex vivo analyses (immunoreactivity, rt-qPCR) of microglia/macrophages-related markers. <b>Results:</b> Repopulation significantly increased <sup>18</sup>F-DPA-714 tracer uptake within the infarct on days 14 (<i>P</i> < 0.001) and 21 (<i>P</i> = 0.002) post ischemia. On day 14, the Iba-1+ cell population showed significantly higher expression of TSPO, CSF-1R and CD68, in line with microglia repopulation. Gene expression analyses on day 14 indicated a significant increase in microglia-related markers (csf-1r, aif1, p2ry12) with repopulation while peripheral cell recruitment-related gene expression decreased (cx3cr1, ccr2), indicative of peripheral recruitment during CSF-1R inhibition. Similarly, uncorrected spleen tracer uptake was significantly higher on day 7 post ischemia with treatment (<i>P</i> = 0.001) and decreased after drug withdrawal. PLX5622-treated mice walked longer distance (<i>P</i> < 0.001) and faster (<i>P</i> = 0.009), and showed stronger forelimbs strength (<i>P</i> < 0.001) than control mice on day 14. <b>Conclusion:</b> This study highlights the potential of <sup>18</sup>F-DPA-714 PET-CT imaging to track microglia/macrophages repopulation after short-term CSF-1R inhibition in stroke.